In the degassing of polyvinylchloride polymer/water mixtures in polymerization autoclaves it is unavoidable, with strongly foaming products, that foam will pass into the following separator. With big polymerization autoclaves, several separators, whose total volume can attain that of the autoclave, are usually arranged behind the autoclave.
The foam, which passes over both in degassing accompanied by the release of the pressure (pressure degassing) and in residual degassing with vacuum (heating by direct injection of steam or jacket heating) is retained in these separators. This efflux is in turn degassed in the separator after the residual degassing in the autoclave is completed by direct injection of steam or jacket heating. Since the efflux is a mixture of various thermally stressed polyvinylchloride polymerisates the efflux retained in residual degassing is again exposed to the high degassing temperature. This leads to a further reduction of the thermostability of the polyvinylchloride polymerisates.
If, for this reason, a lower degassing temperature or a shorter degassing time is selected for the degassing of the efflux in the separators so as not to reduce the thermostability of the product, this results in substantially higher residual vinyl chloride concentrations, which can be 50 times higher than the residual vinyl chloride concentrations of the polymerisate obtained in the autoclave. Another phenomenon in the degassing system is that the volume separated in the separator, and thus the efficiency decreases proportionally to the increasing volume of efflux. This makes another separator necessary which is connected in series or in parallel to the first one. With strongly foaming polyvinylchloride polymerisate/water mixtures it can even happen that the degassing must be stopped to degas the separators and to empty them. This leads to a substantial extension of the total cycle time.
In order to reduce or control the foam buildup in the autoclave and in the resulting efflux, the degassing rate is usually throttled corresponding to the respective requirements.